The present invention relates to a process for gas phase polymerisation of olefins with the aid of a supported chromium oxide based catalyst.
It is known from GB 1,429,174 to prepare olefin polymerisation catalysts by impregnating a refractory oxide carrier with a titanium compound, subjecting the resulting product to a calcination step by heating at a temperature T1 of between 150 and 1200xc2x0 C., adding a chromium compound and subjecting the resulting product to an activation step by heating at a temperature T2 of between 100 and 1200xc2x0 C. Thus, according to GB 1,429,174, two separate thermal treatments (T1, T2), which will be hereinafter referred to as a calcination procedure and an activation procedure are necessary for obtaining the desired catalyst.
It is known from U.S. Pat. No. 3,622,521 to use titanium modified chromium oxide based catalysts for the polymersation of ethylene in slurry. This patent discloses a final activation step of the catalyst which can be carried out in dry air for from 1 to 50 hours using a temperature in the range from 350xc2x0 to 2000xc2x0 F. (176xc2x0 to 1093xc2x0 C.). All the catalysts of U.S. Pat. No. 3,622,521 are prepared using an activation procedure of 5 hours in dry air at 1300xc2x0 F. (704xc2x0 C.).
It is known from U.S. Pat. No. 4,011,382 to prepare ethylene polymers by a gas phase polymerisation process with the aid of a titanium modified chromium oxide based catalyst. This patent describes a final activation step of the catalyst which can e be performed by heating the catalyst in air or oxygen at a temperature of 300xc2x0 to 900xc2x0 C., and preferably at 700xc2x0 to 850xc2x0 C. All the catalysts of U.S. Pat. No. 4,011,382 are prepared using an activation procedure of 8 hours in dry air at either 750xc2x0 C. or 825xc2x0 C.
It is known from EP 0 055 863, to use a chromium supported catalyst for polymerising olefins. All the catalysts disclosed in said patent application are supported on an aluminium phosphate-containing base.
It is also common general knowledge that there is a quasi linear relationship between the temperature of activation and both the activity of the catalyst and the melt index of the ethylene polymer prepared from said chromium oxide catalyst. The higher the temperature of activation, the higher the activity and the melt index.
Therefore, the man in the art knows that in order to properly activate the chromium oxide based catalyst activation temperatures above at least about 500xc2x0 C. should be used.
These calcination/activation procedures applied to the modified support are long and costly. However, as these procedures are necessary to obtain good activity of the final catalyst and a high melt index of the resulting ethylene polymer the man in the art continues to proceed in the same way.
It is therefore an objective of the present invention to provide a process for preparing ethylene (co-)polymers having a high melt index in the presence of a supported chromium oxide based catalyst having reasonably good activity and which does not require these lengthy and/or high temperature calcination/activation procedures.
It has now unexpectedly been found that while said aforementioned quasi linear relationship between the activation temperature and the catalyst activity and melt index of the resulting (co)polymer could systematically be verified in slurry polymerisation, this is not the case when using supported chromium oxide based catalyst in gas phase (co)polymerisation of ethylene.
The present invention makes it possible to avoid or at least mitigate the disadvantages referred to above. In particular, a gas phase polymerisation process has now been found which makes it possible to manufacture polymers having a high melt index with a supported chromium based catalyst showing good activity and which is prepared according to a simple and economical process. Furthermore, the polymers obtained according to the present invention have good stress cracking resistance and a high critical shear rate. As a result the polymers are easy to process. Furthermore, the polymers obtained can have a low volatile content i.e. they give rise to a very small quantity of volatile matter (fumes) during the manufacture of articles. They also can have a high impact strength and a low die swell.
The subject of the invention is therefore a process for the gas phase polymerisation of at least one alpha olefin containing from 2 to 12 carbon atoms, characterised in that the polymerisation is performed with the aid of a chromium oxide based catalyst supported on a granular or microspherical refractory oxide which has been subjected to a sole calcination/activation step consisting of a single thermal treatment performed at a temperature ranging from 200 to 450xc2x0 C. under an oxygen-containing atmosphere.
According to the invention a polymerisation reaction of at least one alpha-olefin is carried out with the aid of such a supported chromium oxide based catalyst.
Another object of the present invention is a method for the preparation of a chromium oxide based catalyst supported on a granular or microspherical refractory oxide for the gas phase polymerisation of olefin(s) characterised in that the supported chromium oxide based catalyst is subjected to a sole calcination/activation step consisting of a single thermal treatment performed at a temperature ranging from 200 to 450xc2x0 C. under an oxygen-containing atmosphere.
Still another object of the present invention is to provide an improved chromium oxide based catalyst supported on a granular or microspherical refractory oxide for the gas phase polymerisation of olefin(s), wherein the catalyst is obtainable by a preparation which is characterised in that the supported chromium oxide based catalyst is subjected to a sole calcination/activation step consisting of a single thermal treatment performed at a temperature ranging from 200 to 450xc2x0 C. under an oxygen-containing atmosphere.
The supported chromium oxide based catalyst contains in most cases from 0.1 to 3% of chromium. According to a preferred embodiment of the present invention, the catalyst is advantageously a titanium or aluminium modified supported chromium oxide based catalyst, most preferably a titanium modified supported chromium oxide based catalyst. For example, the catalyst can be modified with from 0.1 to 8% by weight titanium or 0.1 to 6% by weight of aluminium.
The catalyst is supported on a granular or microspherical refractory oxide such as silica, alumina, zirconia oxide or a mixture or a coprecipitate of these oxides. The support can be obtained by various known processes, especially by precipitation of silicon compounds such as, for example, silica, from a solution of an alkali metal silicate, (or else by coprecipitation of a refractory oxide gel or hydrogel from solutions containing at least two compounds chosen from silicon, titanium, zirconium or aluminium compounds).
The granular support advantageously has a specific (BET) surface of between 200 and 1200 m2/g, a pore volume ranging from 1 to 3.5 ml/g, and can consist of particles which have a diameter of between 20 and 250 xcexcm, preferably between 30 and 150 xcexcm. It advantageously contains hydroxyl functional groups and is preferably free from water at the time of its use during the preparation of the catalyst. For this purpose it can be heated to a temperature ranging e.g. from 100 to 200xc2x0 C.
The catalyst is preferably prepared by a process comprising a first stage during which the support is impregnated with a chromium compound, and a second optional stage during which the product originating from the first stage is impregnated with either a titanium or an aluminium compound. The chromium compound employed can be a chromium oxide, generally of formula CrO3, or a chromium compound which can be converted into chromium oxide by calcining, such as, for example, a chromium nitrate or sulfate, an ammonium chromate, a chromium carbonate, acetate or acetylacetonate or else a tertbutyl chromate.
Titanium compounds which can advantageously be employed are titanium alcoholate such as, for example, titanium tetraisopropylate or titanium tetra-butylate.
Aluminium compounds which can advantageously be employed are for example of the acetyl acetate, acetylacetonate, alkoxy, or alkyl types.
The impregnation of the support with the titanium or the aluminium compound can be performed advantageously just before or during the sole calcination/activation step applied to the catalyst.
The catalyst can also be prepared by a process which consists of a coprecipitation of a gel or hydrogel such as that referred to above in the presence of a chromium compound and of a titanium compound, so that a cogel is formed. comprising, on the one hand, at least one refractory oxide such as silica or alumina, and, on the other hand, a chromium compound and a titanium compound.
Prior to its use the supported catalyst must be subjected to a sole calcination/activation step which consists of a single thermal treatment performed at a temperature ranging from 200 to 450xc2x0 C. under an oxygen-containing atmosphere.
According to the invention said sole calcination/activation treatment of the catalyst must be carried out at a low temperature, in particular at a temperature of between 200xc2x0 C. and 450xc2x0 C., and preferably from 300 to 400xc2x0 C.
According to the invention said sole calcination/activation treatment of the catalyst must be carried out under an oxygen-containing atmosphere, preferably under dry air.
Said single treatment lasts on average between 10 minutes and 12 hours and more particularly between 30 minutes and 8 hours. It can be performed by known means using a non reducing atmosphere. For example, it can be carried out in a fluidised bed activator.
The obtained catalyst can be directly injected into the gas phase polymerisation reactor. It can also be introduced in the form of a prepolymer prepared previously during a prepolymerisation stage. The prepolymerisation stage consists of bringing the catalyst into contact, for example, with ethylene optionally mixed with an alpha-olefin and optionally in the presence of hydrogen. The prepolymerisation stage may be carried out in the presence of an organometallic compound of metal of groups 2 and 13 and optionally 1 and 12 of the periodic classification of the elements.
The gas phase polymerisation of the alpha-olefin may be carried out in a fluidised and/or mechanically stirred bed reactor, according to any known methods. The polymerisation reaction can be carried out at a temperature of between 0 to 120xc2x0 C., preferably between 50 to 110xc2x0 C., and at a total pressure ranging from 0.1 to 5 MPa.
The process according to this invention is particularly suitable for the manufacture of ethylene polymers such as, for example, ethylene homopolymers or ethylene copolymers containing at least one alpha-olefin containing from 3 to 12 carbon atoms such as, for example, 1-butene, 1-hexene or 1-octene. In general, the ethylene copolymers prepared by the process according to this invention contain, in addition to ethylene, less than 10% and in most cases less than 4% and preferably less than 1% by weight of another alpha-olefin containing from 3 to 12 carbon atoms.
The polymers obtained by the process according to this invention may have a relative density ranging from 0.915 to 0.970, preferably ranging from 0.935 and 0.965 and more particularly from 0.940 to 0.960. In most cases, they have a molecular weight distribution, (MWD), measured as the ratio of the weight average molecular weight, MW to the number average molecular weight, MN, of between 5 and 55. In most cases, they have a weight average molecular weight of between 50,000 and 500,000. In general, they contain less than 5 ppm of chromium because of the good activity of the catalyst. Furthermore, they generally have a stress cracking resistance greater than 10 hours and in most cases greater than 15 hours. They also generally have a critical shear rate greater than 800 sxe2x88x921 and in most cases greater than 1000 sxe2x88x921.
The unexpected advantage obtained according to the present invention is the relatively high melt index values of the polymers. As already explained hereinabove and further illustrated in the following (comparative) examples, it is quite surprising that supported chromium oxide based catalysts which have been subjected to a sole calcination/activation treatment at a low temperature produce, when used in gas phase processes, polymers having high melt index values.
When the catalyst is activated at a temperature from 200 to 450xc2x0 C., it is possible to prepare polymers having a high melt index, for example a melt index MI5 higher than 1.5 g/10 minutes. In these ranges the melt index of the polymer increases when the activation temperature decreases.
Such polymers, in particular polymer having a MI5 higher than 1.5 g/10 minutes, the critical shear rate may be greater than 1200 sxe2x88x921.
The polymer also contains a very low proportion of volatile substances. These substances generally represent, on a weight basis, less than 800 and more particularly less than 500 and in most cases less than 400 parts per million (ppm) of the polymer. Furthermore, they generally have a drop strength greater then 2 m and in most cases greater than 2.5 m. They have a low die swell, in particular less than 35 g.
The polymers obtained according to the process of this invention are particularly suitable for the manufacture of objects by extrusion or by blow extrusion.
The following examples illustrate the present invention.
The polymer properties have been measured according to the following procedures.
Method of Determination of the Stress Cracking Resistance
The stress cracking resistance is measured on polymer bottles according to the method of M. J. Cawood and T. J. C. Sleeman (BP Chemicals Ltd. Great Britain), which is described in the journal Polymer Testing 1(1980) pages 191 to 199, except for the fact that the bottles are kept at 50xc2x0 C. instead of 60xc2x0 C. According to this method, the stress cracking resistance is expressed in hours.
Method of Determination of the Volatile Matter Content of a Polymer
According to the invention the volatile matter content of a polymer is determined by measuring the loss in weight of the polymer after it has been kept in an oven at 100xc2x0 C. for 17 hours. The loss in weight is expressed in ppm.
Method of Determination of the Critical Shear Rate
The critical shear rate is determined from a curve which gives the stress imposed on the polymer as a function of the shear rate to which the polymer is subjected, which has been established with the aid of a capillary rheometer which has a die in which the ratio of its length to its diameter is 30. The critical shear rate is determined as the lowest value of the shear rate at which a stress stability is observed. At this value the curve exhibits a point of inflection. The shear stress and rate are defined in ASTM Standard D 3835. A shear rate is expressed in sxe2x88x921.
Fine Particle Content
According to the present invention the fine particle content is the proportion of particles in the polymer having a diameter of less than 125 xcexcm. The content is expressed in weight percent.
Flow Parameter
The flow parameter xe2x80x9cnxe2x80x9d is calculated by the formula n=log (MI21.6/MI5)/log(21.6/5) according to method ASTM D-1238 The melt index is expressed in g/10 minutes.